1. Field of the Invention
This invention relates to an image forming apparatus using the electrophotographic method, and for example, to an image forming apparatus such as a copier, a printer or a facsimile apparatus.
2. Related Background Art
As an image forming apparatus capable of polychromatic printing such as a color printer or a color copier, there has heretofore been proposed a color image forming apparatus which has a plurality of units (process cartridges) provided with at least photosensitive drums and detachably mountable to the main body of an image forming apparatus, and an exposure device for exposing each photosensitive drum charged in conformity with image information and forming an electrostatic latent image, and in which toner images of respective colors formed on the respective photosensitive drums are sequentially superimposed and primary-transferred to an intermediate transfer member, whereafter the toner images of respective colors on the intermediate transfer member are collectively secondary-transferred to a recording material.
Here, a bad image, i.e., an image having color misregistration, occurring when the disposed state of the photosensitive drums is inclined with respect to their regular position will be described with reference to a typical perspective view and a plan view shown in FIG. 12A, consisting of FIGS. 12A1 and 12A2, and 12B, consiting of FIGS. 12B1 and 12B2, respectively, of the accompanying drawings.
Consider a case where as shown in FIG. 12A, a photosensitive drum 501 is inclined by an amount of deviation xcex94x, i.e., an inclination a with respect to an ideal position (alternate long and two short dashes line) in a plane wherein photosensitive drums are juxtaposed. When a laser beam based on image information is scanned on this photosensitive drum 501 by an exposure device, the photosensitive drum is exposed as indicated by a heavy line L(Exp), and at a transfer position, as indicated by a heavy line L(Tr), an amount of deviation 2xcex94x, i.e., an image transferred from the photosensitive drum onto an intermediate transfer member 502, becomes inclined by 2xcex1 which is double the aforementioned inclination xcex1. Further, if the inclinations of a plurality of photosensitive drums differ from one another, the inclinations of the images transferred onto the intermediate transfer member 502 also differ from one another and the positions of the toner images of respective colors on the intermediate transfer member 502 do not become coincident with one another within an allowable range, and become color misregistration or color irregularity.
Also, when as shown in FIG. 12B, the length of the optical path from an exposure device (laser deflector) to the surface of the photosensitive drum 501 differs between the scanning starting side (one lengthwise end side of the photosensitive drum) and the scanning ending side (the other lengthwise end side of the photosensitive drum), even if the light emission timing of the laser beam is controlled to thereby conform the exposure starting positions of the laser beams of respective colors to one another and a modulation frequency for effecting the light modulation of the laser beams is changed to thereby make general magnifications (the width of the image and the width in the main scanning direction) coincident with each other, the inclination xcex2 of the photosensitive drum 501 with respect to the exposure direction leaves an amount of deviation xcex94z at the intermediate position of the image and appears as color misregistration. So, in order to eliminate these inclinations xcex1 and xcex2, that is, to eliminate the amounts of deviation xcex94x and xcex94y relative to the regular position of the photosensitive drum, it is necessary to accurately determine the position at which each photosensitive drum is disposed.
So, heretofore, in order to accurately position the plurality of photosensitive drums 501 in the main body of the image forming apparatus, there has been adopted a method as shown in FIG. 13 of the accompanying drawings wherein the rotary shaft portions 511 of the plurality of photosensitive drums 501 are accurately fixed and supported in engagement holes 604 disposed in a common centering plate 601 comprising a single member, and the reference apertures 602A and 602B of the common centering plate 601 are fitted to positioning pins 603A and 603B provided on a front side plate 600 constituting the frame of the main body of the image forming apparatus to thereby accurately position the photosensitive drums.
In the above-described construction according to the prior art, however, the plurality of photosensitive drums 501 are accurately positioned by the common centering plate 601 which is a single member and therefore, high dimensional accuracy has been required when the engagement holes 604 are formed in the common centering plate 601.
Further, when each photosensitive drum 501 (or process cartridge) is to be interchanged, it is necessary to once detach the common centering plate 601, and in case of the assembly after the interchange, the shaft portions 511 of the plurality of photosensitive drums must be engaged with the engagement holes 604 in the common centering plate 601 at a time, and this has been a factor which makes the work of interchanging the photosensitive drums 501, etc. difficult. Likewise, in the manufacturing process of the image forming apparatus, it has been a factor which makes the assembling work for the photosensitive drum difficult.
It is an object of the present invention to provide an image forming apparatus in which the position of each rotary shaft or each bearing of a first image bearing member and a second image bearing member can be adjusted easily.
It is another object of the present invention to provide an image forming apparatus in which the position of each protruding portion provided on a first unit and a second unit can be adjusted easily.
Further objects of the present invention will become apparent by reading the following detailed description.